I once found an article on the classic displays. I have searched and searched and cannot find the link to that article.

In it, it mentioned the types of driver chips used in the various classic models. I think I have a cathode driver problem in my 65. The 13th digit does not light ('-' sign) and I have eliminated the LED as the source of the problem. I suspect it is the cathode driver (digit drive).

The part is 1820-1226 but I don't have a spare. I have an HP-45 that has an 1820-1061. I'm wondering if that part is interchangeable with the one in the 65. Both are 20 pins and both use the same anode driver 1820-1829.

Can someone point me to the link to the (a) discussion of classic display repair?